1. Field of the Invention
The present invention relates to a mobile communications system and, more particularly, to a method and system for transmitting data when a handover occurs between base stations for a mobile station that is performing data communication.
2. Description of the Related Art
In a mobile communications system or the like employing a packet transmission technique, for example, high-speed packet transmission technique such as HSDPA (High Speed Downlink Packet Access) and EUDCH (Enhanced Uplink Dedicated Channel), significant technical challenges are to prevent loss of data, minimize the duration of a communication interruption and the like at the time of handover (hereinafter, abbreviated as “HO” where appropriate). For example, Japanese Patent Application Unexamined Publication No. 2004-282652 discloses a mobile communications system having a base station controller, in which when a handover occurs between base stations while high-speed packet communication is taking place, the base station controller transfers packet data from the current base station to the handover-target base station, thereby avoiding data loss during handover.
Moreover, at present, the 3GPP LTE (Third Generation Partnership Project, Long Term Evolution) is proposing a handover technique based on data transfer between base stations (see 3GPP TR 23.882 V0.10.0 (2006-01), 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; 3GPP System Architecture Evolution: Report on Technical Options and Conclusions (Release 7)). Hereinafter, brief description will be given of the handover technique using data transfer between base stations, with reference to the accompanying drawings.
FIG. 1 is a network structure diagram schematically showing a general mobile communications system. Here, to simplify the description, it is assumed that two base stations (BTS) 11 and 12 can connect to each other through a network 13, that the base station 11 is the serving base station of a mobile station (MS) 14, and that the mobile station 14 is moving from a cell 1 of the base station 11 into a cell 2 of the target base station 12.
Each of the base stations 11 and 12 can communicate with a gateway (GW) 15 through the network 13. The mobile station 14 transmits and receives data packets to/from the Internet 16 through its serving base station and the gateway 15. Hereinafter, it is assumed that communications between the base stations are performed through an interface XUB and that communications between each base station and the gateway 15 are performed through an interface XU.
1) Downlink Packet Transmission
FIG. 2A is a sequence diagram showing conventional procedures for handover and downlink data transmission using data transfer between base stations. FIG. 2B is a schematic diagram showing an order of transferring unsent packets stored at a serving base station. First, the mobile station 14 evaluates determines the necessity of a handover from the base station 11 to the base station 12 (S20). At this point in time, it is assumed that the base station 11 has received from the gateway 15 packets D(N) to D(N+7) destined for the mobile station 14 (S21), where N is an integer not smaller than zero, and x of a data packet D(x) is a sequence number assigned on the sending side, where the larger x is, the later (the newer, in general) the packet is (the same applies hereinafter).
Here, the serving base station 11 is in a state where packet transmission to the mobile station 14 has been done with up to a packet D(N−1) and where transmission of the packets D(N) to D(N+7) to the mobile station 14 has not started yet, or none of the packets D(N) to D(N+7) has been fully transmitted.
When the mobile station 14 sends a HO request to the serving base station 11 (S22), a handover between the base station 11 and the target base station 12 is decided (S23). When the handover is decided and a HO command, in which a HO activation time AT is set, is sent from the base station 11 to the mobile station 14 (S24), a timer for the HO activation time AT is started (S25). The base station 11 sequentially transfers the stored data packets D(N) to D(N+7) in this order to the base station 12 through the interface XUB.
When the HO activation time AT has expired, the mobile station 14 establishes physical-layer synchronization with the new serving base station 12 (S26). When the synchronization is established, the base station 12 starts sequentially transmitting the transferred packets D(N) to D(N+7) to the mobile station 14. Meanwhile, the gateway 15 updates the serving base station of the mobile station 14 from the base station 11 to the base station 12, based on notifications from the base stations 11 and 12 (S27).
As described above, when a handover occurs, unsent downlink data packets, stored at the base station 11, are transferred to the handover-target base station 12, from which the data packets are transmitted to the mobile station 14. Therefore, loss of data can be suppressed at the time of handover.
2) Uplink Packet Transmission
As to uplink packets to be transmitted from the mobile station 14 to the gateway 15 as well, when a handover occurs, unsent packets are stored at a serving base station and transferred from the serving base station to a target base station. Hereinafter, an uplink-packet case will be described briefly.
FIG. 3 is a sequence diagram showing conventional procedures for handover and uplink data transmission using data transfer between base stations. First, the mobile station 14 transmits uplink packets D(N) to D(N+7) to the current serving base station 11 (S30). It is assumed that these packets are not completely received by the base station 11. For example, when the mobile station 14 transmits one uplink packet D(N) to the base station 11, the mobile station 14 disassembles the packet into a plurality of parts and transmits each part to the base station 11. However, all the parts are not always completely received by the base station 11. If the parts are not completely (or partially) received, the packet D(N) cannot be assembled, which will be referred to as an incompletely (or partially) received packet. Therefore, to transmit packets in order of sequence number, the base station 11 keeps also the fully received packet D(N+1) and subsequent packets without transmitting them. The base station 11 sends a report on the states of these received packets to the mobile station 14, thereby receiving again the packet that has not been completely received.
However, if the mobile station 14, immediately after the incomplete transmission to the serving base station 11, evaluates the necessity of a handover from the base station 11 to the base station 12 (S31) and sends a HO request to the serving base station 11 (S32), the base station 11 stores the received packets and starts HO control. First, when a handover between the base station 11 and the target base station 12 is decided (S33) and a HO command, in which a HO activation time AT is set, is sent from the base station 11 to the mobile station 14 (S34), then a timer for the HO activation time AT is started (S35). The base station 11 sequentially transfers a series of the received packets stored and information about the states of the received packets to the base station 12 through the interface XUB (S36).
Referring to FIG. 3, it is assumed that a fully received packet is represented by a rectangle, and an incompletely (partially) received packet is represented by a trapezoid. Here, the packets D(N), D(N+6) and D(N+7) are assumed to have been incompletely or partially received and are represented by trapezoids. Since the packet D(N) is incomplete, the subsequent packets are also stored at the base station 11 without being transmitted to the gateway 15.
When the HO activation time AT has expired, the mobile station 14 establishes physical-layer synchronization with the new serving base station 12 (S37). When the synchronization with the mobile station 14 is established, the base station 12 sends a report on the states of the received packets to the mobile station 14 (S38). In response to this, the mobile station 14 transmits the packets D(N), D(N+6) and D(N+7) to the base station 12 (S39). When all the packets become complete, the base station 12 transmits the uplink packets D(N) to D(N+7) to the gateway 15 (S40). Meanwhile, the gateway 15 updates the serving base station of the mobile station 14 from the base station 11 to the base station 12, based on notifications from the base stations 11 and 12 (S41).
As described above, when a handover occurs, unsent uplink data packets, stored at the base station 11, are transferred to the handover-target base station 12, from which the packets are then transmitted to the gateway 15 after those corresponding to incomplete packets have been completely received from the mobile station 14. Therefore, loss of data can be suppressed at the time of handover.
However, according to the conventional data transmission procedure shown in FIG. 2A, when a handover is decided, the current serving base station 11 does not transmit the downlink packets D(N) to D(N+7) to the mobile station 14 but transfers them to the base station 12 through the interface XUB. Therefore, even after synchronization has been established between the mobile station 14 and the new serving base station 12, the mobile station 14 cannot receive data at all until all the downlink packets are transferred to the base station 12 and start to be transmitted from the base station 12. During this period, the communication falls in an interruption state. The narrower the bandwidth of the interface XUB between the base stations, the longer time it takes to transfer the packets, and hence the longer the duration of a communication interruption. The duration of a communication interruption is a factor directly related to the quality of radio service, particularly greatly affecting the user's feeling about usability.
Moreover, according to the conventional data transmission procedure shown in FIG. 3, at the time of handover, the unsent uplink data packets stored at the base station 11 are transferred to the handover-target base station 12 and, after the incomplete packets are made complete, transmitted from the base station 12 to the gateway 15. Therefore, even after synchronization has been established between the mobile station 14 and the new serving base station 12, the mobile station 14 practically falls in a state of transmitting no data until all the uplink packets are transferred to the new serving base station 12, which then finishes receiving the packets corresponding to the incomplete packets again and starts transmitting the uplink packets (S40). During this period, the communication falls in an interruption state. The narrower the bandwidth of the interface XUB between the base stations, the longer time it takes to transfer the packets, and hence the longer the duration of a communication interruption. The duration of a communication interruption is a factor directly related to the quality of wireless service, particularly greatly affecting the user's feeling about usability.
To reduce the above-described duration of a communication interruption, the data transfer between the base stations needs to be carried out at as high speed as possible. However, it is undesirable to increase the transfer rate by widening the bandwidth of the interface XUB between the base stations only for this purpose, from the viewpoint of the effective use of network resources. Wireless carriers may also be burdened with higher costs.